The present invention relates to a method of manufacturing a zinc alloy ingot, which has a relatively flat upper surface without cracks or cavities, suitable for use as a replenishment to a molten pool for hot-dip coating a steel strip.
A steel sheet coated with a Zn—Al—Mg alloy plating layer exhibits excellent corrosion-resistance, compared with conventional Zn-coated steel sheets.
There have been proposed various methods so far for formation of the Zn—Al—Mg alloy plating layer. For instance, U.S. Pat. No. 3505043 discloses use of a molten zinc alloy pool containing 3–17 mass % Al and 1–5 mass % Mg. JP 58-177446A discloses use of a molten zinc alloy pool containing 3–25 mass % of Al, 0.05–2 mass % of Mg, 0.005–0.1×Al % of Si and up to 0.02 mass % of Pb. JP 10-226865A discloses a steel sheet hot-dip coated with a plating layer consisting of 4.0–10.0 mass % Al, 1.0–4.0 mass % Mg and the balance being Zn except inevitable impurities. JP 10-306357A discloses a plating layer consisting of 4.0–10.0 mass % Al, 1.0–4.0 mass % Mg, 0.002–0.1 mass % Ti, 0.001–0.045 mass % B and the balance being Zn except inevitable impurities.
The Zn—Al—Mg plating layer is improved in corrosion-resistance and external appearance by controlling a temperature of a molten pool and a cooling rate of a hot-dip coated steel strip, as disclosed in JP 10-226865A and JP 10-306357A.
In order to continuously manufacture a steel strip hot-dip coated with a Zn—Al—Mg plating layer in an industrial scale, a molten pool is periodically replenished by feeding Zn—Al—Mg alloy ingots of the same composition to the molten pool at predetermined intervals in correspondence with consumption of the Zn—Al—Mg alloy.
The Zn—Al—Mg ingot for the purpose shall have a structure without cracks and cavities and an upper surface with a dent sufficiently small in size. If there are cracks or cavities in the ingot, water unfavorably invades into the cracks or cavities during transportation or storage of the ingot. The wet ingot causes very dangerous steam explosion, when it is immersed in the molten pool held at an elevated temperature. The ingots are ordinarily piled up together for transportation or storage. Ingots, which are significantly dented at upper surfaces, can not be piled up together in stationary state. Collapse of piled-up ingots are also very dangerous.
However, a Zn—Al—Mg ingot manufactured by a conventional process is likely to involve cracks and cavities therein, and a big-size dent is often formed on its upper surface.